Fuel cell

ABSTRACT

A fuel cell comprises an anode, a cathode, and a solid electrolyte layer disposed between the anode and the cathode. The cathode includes a perovskite oxide as a main component. The perovskite oxide is expressed by the general formula ABO3 and includes at least one of La and Sr at the A site. The cathode includes a surface region that is within 5 micrometers from the surface opposite the solid electrolyte layer. The surface region contains a main phase configured by the perovskite oxide and a secondary phase that is configured by strontium oxide. The occupied surface area ratio of the secondary phase in a cross section of the surface region is greater than or equal to 0.05% to less than or equal to 3%.

TECHNICAL FIELD

The present invention relates to a fuel cell.

BACKGROUND ART

A typical fuel cell is known to include an anode, a cathode, and a solidelectrolyte layer disposed between the anode and the cathode.

The material used in the cathode is suitably a perovskite oxideincluding at least one of La and Sr at the A site and that is expressedby the general formula ABO₃. (For example, reference is made to JapanesePatent Application Laid-Open No. 2006-32132).

SUMMARY OF INVENTION

However, the fuel cell output may be reduced by repetitive powergeneration. The present inventors have gained the new insight that onecause of a reduction in output results from deterioration of thecathode, and that such deterioration of the cathode is related to theproportion of strontium oxide that is present therein.

The present invention is proposed based on the new insight above, andhas the object of providing a fuel cell that inhibits a reduction inoutput.

Solution to Problem

The fuel cell according to the present invention comprises an anode, acathode, and a solid electrolyte layer disposed between the anode andthe cathode. The cathode contains a main phase configured by aperovskite oxide including at least one of La and Sr at the A site andthat is expressed by the general formula ABO₃, and a secondary phasethat is configured by strontium oxide. The occupied surface area ratioof the secondary phase in a cross section of the cathode is greater thanor equal to 0.05% to less than or equal to 3%.

Advantageous Effects of Invention

The present invention provides a fuel cell that inhibits a reduction inoutput.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross sectional view illustrating a configuration of a fuelcell.

FIG. 2 is a backscattered electron image of a cross section of thesurface region.

FIG. 3 illustrates image analysis results in relation to FIG. 2.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention will be described below makingreference to the figures. Those aspects of configuration in thefollowing description of the figures that are the same or similar aredenoted by the same or similar reference numerals. However, the figuresare merely illustrative, and the actual ratios or the like of therespective dimensions may differ.

Configuration of Fuel Cell 10

The configuration of the fuel cell 10 will be described making referenceto the drawings. The fuel cell 10 is configured as a so-called solidoxide fuel cell (SOFC). The possible configurations of the fuel cell 10include a flat-tubular type, a segmented-in-series type, ananode-supporting type, a electrolyte flat-plate type or a cylindricaltype, or the like.

FIG. 1 is a cross sectional view illustrating a configuration of a fuelcell 10. The fuel cell 10 includes an anode 20, a solid electrolytelayer 30, a barrier layer 40, a cathode 50 and a current collectinglayer 60.

The anode 20 functions as the anode for the fuel cell 10. As illustratedin FIG. 1, the anode 20 includes anode current collecting layer 21 andan anode active layer 22.

The anode current collecting layer 21 is configured as a porous bodythat exhibits superior gas permeability. The constituent materialconfiguring the anode current collecting layer 21 includes use of amaterial that is used in the anode current collecting layer of aconventional SOFC, and for example, includes NiO (nickel oxide)-8YSZ (8mol % of yttria-stabilized zirconia), or NiO—Y₂O₃ (yttria). However,when NiO is included in the anode current collecting layer 21, at leasta portion of the NiO may be reduced to Ni during operation of the fuelcell 10. The thickness of the anode current collecting layer 21 may beconfigured for example as 0.1 mm to 5.0 mm.

The anode active layer 22 is disposed on the anode current collectinglayer 21. The anode active layer 22 is configured as a porous body thatis denser than the anode current collecting layer 21. The constituentmaterial for the anode active layer 22 includes use of a material usedin an anode active layer of a conventional SOFC, and for example,includes NiO-8YSZ. However, when NiO is included in the anode activelayer 22, at least a portion of the NiO may be reduced to Ni duringoperation of the fuel cell 10. The thickness of the anode active layer22 may be configured for example as 5.0 micrometers to 30 micrometers.

The solid electrolyte layer 30 is disposed between the anode 20 and thecathode 50. The solid electrolyte layer 30 in the present embodiment issandwiched between the anode 20 and the barrier layer 40. The solidelectrolyte layer 30 functions to enable permeation of oxygen ions thatare produced by the cathode 50. The solid electrolyte layer 30 isconfigured by a material that is more dense that the anode 20 or thecathode 50.

The solid electrolyte layer 30 may contain ZrO₂ (zirconia) as a maincomponent. In addition to zirconia, the solid electrolyte layer 30 maycontain an additive such as Y₂O₃ (yttria) and/or Sc₂O₃ (scandium oxide).These additives function as a stabilizing agent. The mol compositionratio (stabilizing agent:zirconia) of the stabilizing agent to zirconiain the solid electrolyte layer 30 may be configured to approximately3:97˜20:80. Therefore, the material used in the solid electrolyte layer30 includes 3YSZ, 8YSZ, and 10YSZ, or ScSZ (zirconia stabilized withscandia), or the like. The thickness of the solid electrolyte layer 30for example may be configured as 3 micrometers to 30 micrometers.

In the present embodiment, the term composition X “contains as a maincomponent” composition Y means that composition Y preferably occupies atleast 70 wt % of the total of composition X, and more preferablyoccupies at least 90 wt %.

The barrier layer 40 is disposed between the solid electrolyte layer 30and the cathode 50. The barrier layer 40 inhibits formation of a highresistivity layer between the solid electrolyte layer 30 and the cathode50. The barrier layer 40 is configured by a material that is more densethat the anode 20 or the cathode 50. The barrier layer 40 may include aprincipal component of a ceria based material such as GDC(gadolinium-doped ceria), SDC (samarium-doped ceria), or the like. Thethickness of the barrier layer 40 may be configured for example as 3micrometers to 20 micrometers.

The cathode 50 is disposed on the barrier layer 40. The cathode 50functions as a cathode for the fuel cell 10. The cathode 50 isconfigured as a porous body. The cathode 50 contains a main phaseconfigured by a perovskite oxide including at least one of La and Sr atthe A site and that is expressed by the general formula ABO₃. This typeof perovskite oxide includes (La, Sr)(Co, Fe)O₃ (lanthanum strontiumcobalt ferrite), (La, Sr)FeO₃ (lanthanum strontium ferrite), (La,Sr)CoO₃ (lanthanum strontium cobaltite), La(Ni, Fe)O₃ (lanthanum nickelferrite), (La, Sr) MnO3 (lanthanum strontium manganate), or the like.However, there is no limitation in this regard.

The content ratio of the perovskite oxide in the cathode 50 is greaterthan or equal to 70 wt %. The content ratio of the perovskite oxide inthe cathode 50 is preferably greater than or equal to 90 wt %.

The cathode 50 has a first surface 50S (an example of “a surface”) and asecond surface 50T. The first surface 50S is a surface that is oppositeto the solid electrolyte layer 30. In the present embodiment, since thefuel cell 10 includes the current collecting layer 60, the cathode 50makes contact with the current collecting layer 60 at the first surface50S. That is to say, in the present embodiment, the first surface 50S isthe interface between the cathode 50 and the current collecting layer60. The second surface 50T is the surface on the solid electrolyte layer30 side. In the present embodiment, since the fuel cell 10 includes thebarrier layer 40, the cathode 50 makes contact with the barrier layer 40at the second surface 50T. That is to say, in the present embodiment,the second surface 501 is the interface between the cathode 50 and thesecond surface 50T.

The current collecting layer 60 is disposed on the cathode 50 (surfaceregion 51). Although there is no particular limitation in relation tothe thickness of the current collecting layer 60, it may be configuredas 30 micrometers to 500 micrometers. The current collecting layer 60can be configured by the perovskite composite oxide expressed by thecomposition formula (1) below. However, there is no limitation in thisregard. The material used in the current collecting layer 60 ispreferably a material that exhibits a smaller electrical resistance thanthe material used in the cathode 50.La_(m)(Ni_(1-x-y)Fe_(x)Cu_(y))_(n)O_(3-δ)  (1)

A substance other than La may be contained in the A site of compositionformula (1), and a substance other than Ni, Fe or Cu may be contained inthe B site. In composition formula (1), m and n are greater than orequal to 0.95 and less than or equal to 1.05, x (Fe) is greater than orequal to 0.03 and less than or equal to 0.3, y (Cu) is greater than orequal to 0.05 and less than or equal to 0.5, and δ is greater than orequal to 0 and less than or equal to 0.8.

Configuration of Cathode 50

The cathode 50 includes a surface region 51 and an inner region 52. Thesurface region 51 is disposed on the inner region 52. The surface region51 is a region within 5 micrometers from the first surface 50S. In thepresent embodiment, since the fuel cell 10 includes a current collectinglayer 60, the surface region 51 is a region of the cathode 50 within 5micrometers from the current collecting layer 6. The inner region 52 isa region other than the surface region 51 of the cathode 50. In thepresent embodiment, since the fuel cell 10 includes a barrier layer 40,the inner region 52 is sandwiched between the surface region 51 and thebarrier layer 40. The thickness of the inner region 52 in the directionof thickness may be configured as 5 micrometers to 300 micrometers.

The first surface 50S may be determined based on a line of rapid changein a concentration distribution of a predetermined component whenmapping the component concentration in a cross section that is parallelto the direction of thickness in the cathode 50 and the currentcollecting layer 60. More specifically, the first surface 50S is takento be the line at which the concentration of an element that issubstantially included in only one of the cathode 50 or the currentcollecting layer 60 takes a value of 10% of the maximum concentration inan inner portion of that component. The second surface 50T may bedetermined based on a line of rapid change in the concentrationdistribution of a predetermined component when mapping the componentconcentration in a cross section that is parallel to the direction ofthickness in the barrier layer 40 and the cathode 50. More specifically,the second surface 50T is taken to be the line at which theconcentration of an element that is substantially included in only oneof the barrier layer 40 or the cathode 50 takes a value of 10% of themaximum concentration in an inner portion of that component.

The surface region 51 contains a main component of a perovskite oxideincluding at least Sr at the A site and that is expressed by the generalformula ABO₃. The occupied surface area ratio of the main phase that isconfigured by the perovskite oxide in the cross section of the surfaceregion 51 may be configured as greater than or equal to 97% and lessthan or equal to 99.5%.

The surface region 51 contains strontium oxide (SrO) as a secondarycomponent. The occupied surface area ratio of the secondary phaseconfigured by SrO in the cross section of the surface region 51 isgreater than or equal to 0.05% and less than or equal to 3%. Since theinactive portion in the inner portion of the surface region 51 isreduced by a configuration of the occupied surface area ratio of thesecondary phase to less than or equal to 3%, it is possible to inhibitthe progression of deterioration of the cathode during electricalconduction resulting from a reaction between the secondary phase and themain phase. Furthermore, since the backbone of the porous structure canbe strengthened and sintering characteristics of the surface region 51can be enhanced by a configuration of the occupied surface area ratio ofthe secondary phase to greater than or equal to 0.05%, it is possible toinhibit microscopic structural changes to the surface region duringelectrical conduction. Consequently, it is possible to enhance thedurability of the cathode 50.

In the present embodiment, the feature of “occupied surface area ratioof substance Z in a cross section” means the proportion in the totalsurface area of substance Z relative to the total surface area includingthe pores and the solid phase. The method of calculating the occupiedsurface area ratio will be described below.

The average equivalent circle diameter of the secondary phase in thecross section of the surface region 51 is preferably greater than orequal to 10 nm and less than or equal to 500 nm. A further reduction tothe deterioration rate of the cathode 50 is enabled in this manner. Theequivalent circle diameter is the diameter of a circle that has the samesurface area as the secondary phase in an analysis image that isanalyzed using a field emission scanning electron microscope (FE-SEM) asdescribed below. The average equivalent circle diameter is the value ofthe arithmetic average of greater than or equal to 20 equivalent circlediameters for the secondary phase. The greater than or equal to 20secondary phase samples that are the object of equivalent circlediameter measurement are preferably selected in an arbitrary manner fromfive or more positions on an FE-SEM image.

The constituent element (for example, La, Co, or the like) in the mainphase may be in solid solution in the secondary phase. Furthermore, thesecondary phase may include a minute amount of impurities other thanSrO.

In addition to the main phase and the secondary phase, the surfaceregion 51 may include a third phase configured by a perovskite oxidethat is expressed by the general formula ABO₃ and that is different fromthe main phase, and by an oxide of the constituent elements of the mainphase, or the like. The oxide of the constituent elements of the mainphase include for example SrO, (Co, Fe)₃O₄, and Co₃O₄ or the like. (Co,Fe)₃O₄ includes Co₂FeO₄, Co_(1.5)Fe_(1.5)O₄, and CoFe₂O₄, or the like.

The occupied surface area ratio of the third phase in the cross sectionof the surface region 51 may be configured to less than or equal to 10%.In this manner, microcracks after thermal cycle testing as well as afterfiring can be inhibited. Thermal cycle testing refers to testing thatincludes 10 repetitions of a cycle of maintaining a reducing atmosphereby supplying Ar gas and hydrogen gas (4% relative to Ar) to the anode,and increasing the temperature from ambient temperature to 800 degreesC. over 2 hours followed by reducing the temperature to ambienttemperature over 4 hours.

The inner region 52 includes a main phase configured by a perovskiteoxide including at least Sr at the A site and that is expressed by thegeneral formula ABO₃. The occupied surface area ratio of the main phasein a cross section of the inner region 52 is greater than or equal to95%. The inner region 52 may include a secondary phase configured bySrO. In addition to the main phase, the inner region 52 may include athird phase configured by a perovskite oxide as described above, anoxide of the constituent elements of the main phase, or the like.

Method of Calculation of Occupied Surface Area

The method of calculation of the occupied surface area ratio in a crosssection of the surface region 51 will be described making reference tothe figures. In the following description, although a method ofcalculation of the occupied surface area ratio of the secondary phasewill be described, the occupied surface area ratio of the main phase orthe third phase may be calculated in the same manner.

(1) Backscattered Electron Image

FIG. 2 illustrates an example of a backscattered electron image of across section of the surface region 51 enlarged with a magnification of10,000 times by FE-SEM using a backscattered electron detector. FIG. 2illustrates a cross section of the cathode 50 that contains (La, Sr)(Co,Fe)O₃ as a main component. The backscattered electron image in FIG. 2 isobtained by an FE-SEM (model: ULTRA55) manufactured by Zeiss AG(Germany) with a working distance setting of 2 mm, and an accelerationvoltage of 1 kV. The cross section of the surface region 51 ispreprocessed with polishing with precision machinery followed by an ionmilling process performed using an IM4000 manufactured by HitachiHigh-Technologies Corporation.

In the backscattered electron image illustrated in FIG. 2, the contrastof the main phase (La, Sr)(Co, Fe)O₃ differs from that of the secondaryphase SrO, and the main phase is displayed as “faint gray”, thesecondary phase as “gray” and the pores as “black”. In this manner,three values assigned in relation to the contrast can be realized bycategorizing the luminosity of the image into 256 gradations. The mainphase, secondary phase and third phase can be identified from thecontrast of the backscattered electron image.

(2) Analysis of Backscattered Electron Image

FIG. 3 illustrates image analysis results using HALCON image analysissoftware produced by MVTec GmbH (Germany) in relation to thebackscattered electron image illustrated in FIG. 2. In FIG. 3, thesecondary phase is represented as the white area enclosed by a blacksolid line.

(3) Calculation of Occupied Surface Area

The total surface area of the secondary phase in the white areas iscalculated with reference to the analysis image in FIG. 3. Next, theproportion of the total surface area of the secondary phase relative tothe surface area (including the pores and the solid phase) in the totalbackscattered electron image is calculated. The proportion of the totalsurface area of the secondary phase calculated in this manner is takento be the occupied surface area ratio of the secondary phase in thesurface region 51.

Material of Surface Region 51

The cathode material used to configure the surface region 51 is amixture including a main phase configured by a perovskite oxide and asecondary phase of SrO. SrO may be configured as a mixture of strontiumcarbonate, strontium hydroxide, and strontium nitrate.

The occupied surface ratio of the secondary phase in the surface region51 may be adjusted by adjusting the added amount of material powdercontaining SrO.

Adjusting the particle size of the material powder containing SrOenables an adjustment of the average equivalent circle diameter of thesecondary phase in the surface region 51. An accurate classificationthat includes an upper limiting value and a lower limiting value of theparticle diameter is possible by use of an air classifier to adjust thegrain size of the material powder containing SrO. When the particle sizeof the material powder containing SrO has a coarse configuration, theaverage equivalent circle diameter of the secondary phase can beconfigured to be large, and when the particle size is fine, the averageequivalent circle diameter of the secondary phase can be configured tobe small. Furthermore, when the particle size distribution of thematerial powder containing SrO is large, the average equivalent circlediameter of the secondary phase can be configured to be large, and whenthe particle size distribution is small, the average equivalent circlediameter of the secondary phase can be configured to be small.

Method of Manufacturing Fuel Cell 10

Next, an example will be described of a manufacture method for the fuelcell 10.

Firstly, a green body for the anode current collecting layer 21 isformed by molding an anode current collecting layer material powderusing a die press molding method.

Then, a slurry for the anode active layer is formed by adding PVA(polyvinyl alcohol) as a binder to a mixture of a pore forming agent(for example, PMMA) and the anode active layer material powder. Theslurry for the anode active layer is printed onto the green body of theanode current collecting layer 21 using a printing method or the like tothereby form a green body for the anode active layer 22. The green bodyfor the anode 20 is formed as described above.

Next, a slurry for the solid electrolyte layer is prepared by mixingterpineol and a binder with a solid electrolyte layer material powder.The slurry for the solid electrolyte layer is coated onto the green bodyof the anode active layer 22 using a printing method or the like tothereby form a green body for the solid electrolyte layer 30.

Next, a slurry for the barrier layer is prepared by mixing terpineol anda binder with a barrier layer material powder. The slurry for thebarrier layer is coated onto the green body of an intermediate layer 40using a printing method or the like to thereby form a green body for thebarrier layer 40.

Next, the green bodies respectively for the anode 20, the solidelectrolyte layer 30 and the barrier layer 40 are fired (1350 to 1450degrees C., 1 to 20 hours) to form the anode 20, the solid electrolytelayer 30 and the barrier layer 40.

Then, the perovskite oxide material including at least one of La and Srat the A site as described above and that is expressed by the generalformula ABO₃, water and a binder are mixed in a ball mill for 24 hoursto prepare a slurry for the inner region.

Then the slurry for the inner region is coated onto the barrier layer 40using a printing method or the like to thereby form a green body for theinner region 52.

The material for the surface region 51 as described above (mixedmaterial containing perovskite oxide material as a main component andSrO as a secondary component), water and a binder are mixed in a ballmill for 24 hours to prepare a slurry for the surface region. At thattime, the occupied surface area ratio of the secondary phase in thesurface region 51 after firing can be controlled by adjusting the mixedamount of SrO.

Then the slurry for the surface region is coated onto the green body forthe inner region 52 using a printing method or the like to thereby forma green body for the surface region 51.

Then the material for the current collecting layer 60 as describedabove, water and a binder are mixed to prepare a slurry for the currentcollecting layer.

Then the slurry for the current collecting layer is coated onto thegreen body for the surface region 51 to thereby form a green body forthe current collecting layer 60.

The green body for the cathode 50 is fired (1000 to 1100 degrees C., 1to 10 hours) to form the cathode 50.

Other Embodiments

The present invention is not limited to the above embodiment, andvarious changes or modifications may be added within a scope that doesnot depart from the scope of the invention.

In the above embodiment, although the fuel cell 10 includes the currentcollecting layer 60, the current collecting layer 60 may be omitted. Inthis configuration, the first surface 50S of the cathode 50 becomes theouter surface of the fuel cell 10, and the surface region 51 of thecathode 50 is a region within 5 micrometers of the outer surface.

Although the cathode 50 includes the barrier layer 40, the barrier layer40 may be omitted. In this configuration, the second surface 50T of thecathode 50 makes contact with the solid electrolyte layer 30, andtherefore the inner region 52 of the cathode 50 is sandwiched betweenthe surface region 51 and the solid electrolyte layer 30.

Although the barrier layer 40 is configured with a monolayerconfiguration, a laminated structure may be provided in which a densebarrier layer is laminated (randomly) with a porous barrier layer.

Although the examples of a fuel cell according to the present inventionwill be described below, the present invention is not thereby limited tothe following examples.

Examples

Preparation of Samples No. 1 to No. 12

A fuel cell according to Samples No. 1 to No. 12 is prepared asdescribed below.

Firstly, a mixed powder is prepared by drying a slurry of a mixture ofIPA and a compounding powder of a pore-forming agent (PMMA), Y₂O₃ powderand NiO powder in a nitrogen atmosphere.

Next, uniaxial pressing (compaction pressure 50 MPa) is applied to themixed powder to form a plate of 30 mm length×30 mm width and a thicknessof 3 mm. A green body for the anode current collecting layer is preparedby further consolidation of the plate by use of a CIP (compactionpressure: 100 MPa).

Next, the slurry formed from a mixture of IPA and a compounding powderof PMMA and NiO-8YSZ is coated onto the green body for the anode currentcollecting layer.

Next, a slurry for the solid electrolyte layer is prepared by mixingterpineol and a binder with 8YSZ. Then the slurry for the solidelectrolyte layer is coated onto the green body of the anode to therebyform a green body for the solid electrolyte layer.

Then a GDC slurry is prepared, and the GDC slurry is coated onto thegreen body for the solid electrolyte layer to thereby prepare a greenbody for the barrier layer.

Next, the green bodies respectively for the anode, the solid electrolytelayer and the barrier layer are fired (1450 degrees C., 5 hours) to formthe anode, the solid electrolyte layer and the barrier layer.

Next, a slurry for the inner region is prepared by mixing terpineol anda binder with a perovskite oxide material (a main component for thecathode) as shown in Table 1. The slurry for the inner region is coatedonto the barrier layer to thereby prepare a green body for the innerregion.

Next, a surface region material is prepared by adding a powder of amaterial containing SrO (secondary component of surface region) to thepowder of a perovskite oxide material (a main component for the surfaceregion) as shown in Table 1. At that time, the addition amount of SrO isadjusted in each sample so that the occupied surface area ratio of thesecondary phase (SrO) in the cross section of the surface region takesthe values shown in Table 1. Furthermore, the particle size of SrO isadjusted so that the average equivalent circle diameter of the secondaryphase takes the values shown in Table 1.

Next, a slurry for the surface region is prepared by mixing terpineoland a binder with a surface region material. The slurry for the surfaceregion is coated onto the green body for the inner region to therebyprepare a green body for the surface region. The coating amount at thattime is adjusted so that the thickness of the surface region afterfiring is less than or equal to 5 micrometers.

Next, a slurry for the current collecting layer is prepared by mixingwater and a binder with an La(NiFeCu)O₃ powder. The slurry for thecurrent collecting layer is coated onto the green body for the surfaceregion to thereby form a green body for the current collecting layer.

The green body for the inner region, surface region and currentcollecting layer are fired (1000 to 1100 degrees C., 1 hour) to form thecathode and current collecting layer.

Measurement of Occupied Surface Area Ratio

After polishing of the cathode in each sample with precision machinery,ion milling processing is performed using an IM4000 manufactured byHitachi High-Technologies Corporation.

A backscatter electron image of the cross section of the surface regionenlarged with a magnification of 10,000 times by a FE-SEM using abackscatter electron detector is acquired. FIG. 2 is a backscatterelectron image of the surface region cross section of Sample No. 5.

Then, an analysis image is acquired by analyzing the backscatterelectron image for each sample using HALCON image analysis softwareproduced by MVTec GmbH (reference is made to FIG. 3). The secondaryphase configured by SrO is illustrated in FIG. 3 by the white areas.

Then, the occupied surface area ratio of the secondary phase to thetotal surface area (including gas phase and solid phase) in thebackscatter electron image is calculated. The calculation results forthe occupied surface area ratio of the secondary phase are shown inTable 1.

Average Equivalent Circle Diameter of Secondary Phase

An analysis image of the backscatter electron image as described aboveis acquired at five positions in the cross section of the surface regionof the cathode to thereby calculate the average equivalent circlediameter of the twenty secondary phase that are arbitrarily selectedfrom the five analysis images. The calculation results for the averageequivalent circle diameter of the secondary phase are shown in Table 1.

Durability Testing

Samples No. 1 to No. 12 are heated to 750 degrees C. while supplyingnitrogen gas to the anode side and air to the cathode side. Whenreaching a temperature of 750 degrees C., hydrogen gas is supplied tothe anode to perform a reduction process for three hours.

Next, a voltage drop rate per 1000 hours is measured as a deteriorationrate. The output density at a rated current density value of 0.2 A/cm²at a temperature of 750 degrees C. is used. The measurement results aresummarized in Table 1. In the present embodiment, a sample having adeterioration rate of less than or equal to 1.5% is evaluated as havinga low deterioration state.

The presence or absence of cracks in the cathode inner portion isobserved by electron microscope observation of a cross section of thecathode after durability testing. Table 1 denotes samples confirmed tohave a crack of greater than or equal to 5 micrometers as “YES”, anddenotes samples confirmed to have a crack of less than 5 micrometers as“YES (insignificant)”. The observation results are shown in Table 1.

TABLE 1 Occupied Surface Equivalent Circle Area Ratio of Diameter ofDeterioration Main Component Secondary Phase Secondary Phase RatePresence/Absence Sample of Cathode (SrO) (SrO) (nm) (%) of microcracksEvaluation 1 (La, Sr)FeO₃ 0.02 5 2.2 YES X 2 (La, Sr)(Co, Fe)O₃ 0.03 32.1 YES X 3 (La, Sr)(Co, Fe)O₃ 0.05 5 1.4 YES ◯ (INSIGNIFICANT) 4 (La,Sr)(Co, Fe)O₃ 0.15 10 1.1 NO ⊚ 5 (La, Sr)(Co, Fe)O₃ 0.34 64 0.2 NO ⊚ 6(La, Sr)FeO₃ 0.65 125 0.1 NO ⊚ 7 (La, Sr)(Co, Fe)O₃ 1.1 180 0.3 NO ⊚ 8(La, Sr)FeO₃ 1.5 120 0.4 NO ⊚ 9 (La, Sr)(Co, Fe)O₃ 2.1 360 0.7 NO ⊚ 10(La, Sr)FeO₃ 2.6 500 0.8 NO ⊚ 11 (La, Sr)(Co, Fe)O₃ 3.0 580 1.3 YES ◯(INSIGNIFICANT) 12 (La, Sr)(Co, Fe)O₃ 3.8 610 2.1 YES X (INSIGNIFICANT)

As shown in Table 1, samples in which the occupied surface area ratio ofthe secondary phase (SrO) in the surface region is greater than or equalto 0.05% and less than or equal to 3% exhibit a reduction in thedeterioration rate of the cathode to less than or equal to 1.5%, andinhibit the formation of microcracks. This feature is due to that factthat deterioration of the cathode is inhibited by reducing the inactiveportion of the surface region inner portion by configuring the occupiedsurface area ratio of the secondary phase to less than or equal to 3%,and strengthening the backbone of the porous structure as a result ofimproving the sintering characteristics of the cathode due to aconfiguration in which the occupied surface area ratio of the secondaryphase to greater than or equal to 0.05%.

In samples in which the average equivalent circle diameter of thesecondary phase as shown in Table 1 is greater than or equal to 10 nmand less than or equal to 500 nm, a further inhibition on the formationof microcracks in the inner portion of the surface region is enabled.

Industrial Applicability

The invention claimed is:
 1. A fuel cell comprising: an anode, a cathode containing a perovskite oxide as a main component, the perovskite oxide expressed by the general formula ABO₃, the A site including at least one selected from the group consisting of La and Sr, and the B site including at least one selected from the group consisting of Fe, Co, Mn and Ni, and a solid electrolyte layer disposed between the anode and the cathode, wherein: the cathode includes a surface region which is within 5 micrometers from a surface opposite the solid electrolyte layer, the surface region contains a main phase comprising the perovskite oxide and a secondary phase comprising strontium oxide, an occupied surface area ratio of the strontium oxide in a cross section of the surface region is greater than or equal to 0.05% and less than or equal to 3%, the cross section of the surface region being parallel to a thickness direction of the cathode, and an average equivalent circle diameter of the strontium oxide in the cross section of the surface region is greater than or equal to 10 nm and less than or equal to 500 nm.
 2. The fuel cell according to claim 1, wherein a current collecting layer is disposed on the surface region of the cathode. 